At present new high-sensitive image sensors having wide spectral recording capabilities and many picture elements ("pixels") are available on the market for the field of spectral analysis. One category of these image sensors is the CCD-sensor (Charge Coupled Device sensor). Modem electronic techniques offer both rapid data conversion (A/D-conversion) and rapid processing of large amounts of information that image sensors generate. In order to take advantage of these techniques for optical spectral analysis, it is required that the optical arrangements preceding the sensor can be optimzed in such a way that the techniques are fully utilized in the spectral analysis. This is not the case with the techniques available today.
The use of diffraction gratings for wavelength dispersion, i.e. separation of optical radiation into wavelength components, is a well known technique. Diffraction gratings are very effective optical elements for carrying out wavelength dispersion. However, diffraction gratings have the disadvantage that spectra from several spectral orders result in ambiguities in the analysis of a spectrum.
Modern Optical sensors are sensitive to optical radiation simultaneously in the wavelength range from vacuum ultra violet ("VUV"), at wavelengths below 200 nm, to the near infrared ("NIR"), at wavelengths above 700 nm. In order to use modern optical sensors, in a single instrument, for simultaneous recording of the whole spectral range, the problem of ambiguities from other spectral orders has to be solved. One solution to the problem is to place several optical filters suitably in the vicinity of the focal plane of the optical instrument. Through this arrangement one can in principle extend the recording capability in a single spectral order. However, diffraction gratings manufactured either holographically or through ruling show a variation of the efficiency by which the optical radiation is dispersed. This diffraction efficiency, i.e. the ratio between the intensity of the dispersed light and the intensity of the incident light at a certain wavelength, has a maximum value at a wavelength given generally in the first spectral order.
The wavelength, called "the blaze wavelength", is a quantity characteristic for the diffraction grating, and depends on the distance between the grating grooves, the shape of the grating grooves, and the polarization of the optical radiation. In selecting a diffraction grating the blaze wavelength is chosen within the wavelength range of the instrument. When the wavelength to be recorded diverges considerably from the blaze wavelength, the diffraction efficiency of the grating is substantially reduced.
The method discussed above to filter disturbing wavelengths from other spectral orders to decrease the degree of ambiguity in a single spectral order results in a considerable reduction in the efficiency of the spectral recording at wavelengths that diverge from the blaze wavelength. This reduction in efficiency makes it difficult to cover a large wavelength range in one single spectral order of a diffraction grating.
In order to avoid the reduction in efficiency, G. R. Harrison in J. Opt. Soc. Am. No. 39 (1949) page 522, proposed using diffraction gratings with coarse rulings, or gratings with a distance d between the grooves considerably larger than the wavelength .lambda. to be measured. The gratings, called "echelle gratings", are produced having a step shape of the cross section of the grooves with one of the groove facets plane and highly reflecting. The angle between the normal to the reflecting groove facet and the normal to the grating surface is called the "blaze angle .theta.". The blaze angle .theta. for an echelle grating is typically 60.degree.. Very high diffraction efficiencies over a very broad wavelength range, such as from the ultra violet (UV) to the near infrared (NIR) are possible using echelle gratings. As shown below, echelle gratings can be used to disperse radiation in spectral orders m from m=1 to m&gt;100 with conserved high diffraction efficiency in the vicinity of the blaze wavelengths .lambda..sub.m.sup.0 in each spectral order.
These special features of echelle gratings can be utilized only if the spectra, produced in the different spectral orders, i.e. "the order spectra", can be separated from each other at the focal surface in the recording. In a spectral apparatus that records the spectrum sequentially such as a monochromator, the separation is possible using for example filters placed in front of the entrance aperture of the monochromator. However, if a large wavelength range is to be measured simultaneously in a single exposure, such as using a photographical recording or a CCD sensor recording, the order spectra in the focal plane need to be separated from each other. In order to separate the order spectra in the focal plane, an additional wavelength dispersing optical element, called an "order sorter," is introduced in the ray path before the focal plane. The direction of the wavelength dispersion must then be made orthogonal to the direction of dispersion of the diffraction grating.
Two main means to achieve order sorting are refraction of the radiation as, for example by a prism or diffraction as, for example by a grating. The diffraction means produce separation of the order spectra of the echelle grating in several spectral orders, making the simultaneous recording of large wavelength ranges impossible due to disturbances from the spectral orders of the grating order sorter.
Such order sorters are found in SE-C-359 648 and are also described by A. Danielsson and P. Lindblom in Physica Scripta No. 5 (1972) page 227 and by S. Engman, P. Lindblom and B. Sandberg in Physica Scripta No. 24 (1981) page 965. Order sorters of the type discussed, have either a single diffraction grating or a combination of a diffraction grating and a prism. The order sorting means discussed above were designed for detection with an image tube with sequential read-out. Filters could be used during read-out for the purpose of separating disturbances from other orders of the grating in the order sorter. These order sorters are impractical for simultaneous recording of the whole required wavelength.
It is known that for an order sorter to separate the order spectra uniformly, meaning that they have a constant mutual distance in the focal plane, the wavelength dependence of the angular dispersion of the order sorter needs to be inversely proportional to the square of the wavelength. This is expressed by equation (12) in the introduction to the theoretical section below. For a refractive, i.e. prismatic order sorter, used in a large wavelength range such as VUV to NIR, there exist no optical materials with dispersive properties that would result in the required dependence.
Consequently the order spectra on the focal surface are distributed very non-uniformly as shown by P. Lindblom et Al. in the prints mentioned above.
The shortest distance between the order spectra on the focal surface determines the height of the entrance aperture of the spectral apparatus. As the detector, for example a CCD sensor, has a restricted size, the non-uniform distribution of the order spectra results in a restriction of the sensitivity of the spectral apparatus, i.e. a restriction of the amount of optical radiation that it can detect. Very large wavelength ranges are then impossible to record simultaneously. The surface of the sensor is also utilized inefficiently as a result of the non-uniform distribution.
The features discussed above highlight special demands that order sorting of the spectral orders puts on the optical image of the entrance aperture on the focal plane. The height of the entrance aperture must be chosen so that the corresponding height of the image of the entrance aperture on the focal plane is smaller than the shortest distance between the order spectra, or the orders spectra will mix in the recording, i.e. one order spectrum receives a "cross talk" from the spectral signal of the neighbouring order spectra. As spectroscopic applications generally require that the spectrum can he measured without disturbances in intensity ranges where the weakest signal is 1/10000 or less than the strongest signal, even a small cross talk from one order spectrum to another is very disturbing. This feature puts very high demands on the quality of the optical image of the entrance aperture, namely that the image must be stigmatic, meaning that any point at the entrance aperture is imaged substantially on a point in the focal plane.
The requirement that the image be stigmatic differs from normal requirements of spectroscopic equipment, that it is sufficient if a point at the entrance aperture is imaged on a line oriented orthogonally to the dispersing direction of the grating, in the focal plane. If such non-stigmatic, or astigmatic imaging, is applied in a spectral apparatus with an a echelle grating, the optical radiation from neighboring order spectra will be mixed and produce cross talk at read-out, even if the height of the entrance aperture is reduced. As discussed above, small cross talk between order spectra on the sensor surface is unacceptable in spectral analysis.
The non-uniform distribution of the order spectra and the astigmatic imaging of the entrance aperture are both eliminated in the spectral apparatus of the present invention.
Another aberration of the optical image that is disturbing in optical spectral instruments is called "coma," which appears as an asymmetric broadening of the optical image. In a spectral apparatus the broadening of the optical image results in a deformation of the optical image of the entrance aperture in an isolated wavelength (spectral line), resulting in a reduction of the spectral resolution. This aberration is also reduced or eliminated through the present invention.
The detector unit of spectral instruments is often expensive. Its light sensitive surface, which normally has a rectangular shape also has a restricted size. In relation to initially specified spectral properties, it is generally hard to make a choice of the main parameters of the spectral apparatus so that this surface is utilised in an optimal way. This difficulty is also eliminated or reduced by the present invention.